1. Infrared Radiation: Molecular Vibrations1

2. Do Molecules Absorb Energy
The bonds in molecules absorb energy. Have you ever wondered why the sky is blue? Or why it is red during a sunset? Do you know why are apples are red and leaves green? Answer: It is because they are made up of different molecules with different bonds and functional groups….see the next slide for a more detailed answers.

3. Infrared Absorbance
Sample

4. Bonds IR Absorption Spectra
When molecules are hit by IR most of the light passes by (transmittance) but some is absorbed by the bonds.
BOND
WAVENUMBER
SHAPE OF PEAK
C-O
STRONG AND LONG (OUT OF RANGE, FINGERPRINT)
C=O
LONG AND NARROW, POINTY
C-H
LONG AND LARGE, POINTY
O-H (ACIDS)
WIDE, BROAD, DEEP
O-H (ALCOHOLS)

5. A bond must have a dipole to be detected in the
IR spectrum.
NOTE: Non polar Bonds (O2) are not detected.

6. IR SPECTRUM OF A CARBONYL COMPOUND
carbonyl compounds (C=O bond) show a sharp, strong 1700

7. A Ketone IR Spectrum
=>
Chapter 12

8. IR SPECTRUM OF AN ALCOHOL
• alcohols (O-H bond) show a broad absorption between 3200 and 3600 cm-1

9. IR SPECTRUM OF A CARBOXYLIC ACID
-COOH
3000
C=O
1700
• carboxylic acids show a very broad 3000 cm-1
• they also show a strong absorption around 1700 cm-1 due to C=O bond

10. Infrared Spectroscopy
4-phenylbutyric acid
Carboxylic Acids:
Gives the messiest of IR spectra
C=O band occurs between cm-1
The highly dissociated O-H bond has a broad band from cm-1 covering up to half the IR spectrum in some cases
(w – m) br
(s)
(s)

11. Infrared Spectroscopy Octane
Alkanes – combination of C-C and C-H bonds
C-C stretches and bends cm-1
CH2-CH2 bond cm-1
CH2-CH3 bond cm-1
sp3 C-H between cm-1
(w – s)
(m)

12. Infrared Spectroscopy 1-Octene
Alkenes – addition of the C=C and vinyl C-H bonds
C=C stretch at cm-1 weaker as substitution increases
(w – m)
(w – m)

13. Infrared Spectroscopy 1-Octyne
Alkynes – addition of the C=C and vinyl C-H bonds
C≡C stretch cm-1; strength depends on asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes
C-H for terminal alkynes occurs at cm-1
Internal alkynes ( R-C≡C-R ) would not have this band!
(w-m)
(m – s)

14. Infrared Spectroscopy 2-aminopentane
Amines - Primary
Shows the –N-H stretch for NH2 as a doublet between cm-1
(w)
(w)

15. Examples of Real Spectra
Now we will look at some actual spectra To see the functional group areas of absorbance

16. =>
Chapter 12

17. An Infrared Spectrum
C-H
3000
FOCUS on the molecules F.G (functional groups) OH, C=O

18. Pentanoic Acid
The alcohol of the carboxylic acid and C-H bond are
overlapping somewhat. The C=O is at 1700.

19. FINGERPRINT REGION

20. FINGERPRINT REGION
THE NAME SAYS IT ALL, YOU TELL ME WHAT GOOD ARE FINGERPRINTS TO POLICE?
OK , SEE NEXT SLIDE….you will see the 2 molecules are isomers and
Very similar in the diagnostic FG region, but they cannot be superimposed
In the finger print region to be an exact match

21. Superimposition
Were the last 2 isomers an exact match? No, so just like fingerprints, if they do not match it is not the same compound.

22. The functional group
Area
The fingerprint
region

23. MORE REGIONS TO STUDY
Examples for you to try

24. What compound is the most likely choice?
Is this:
A) Phenol B) Toluene C) Cyclohexanone D)Cyclohexane
Answers on next slide

25. Aliphatic ketone C-H It is choice C….and the reason, click to find out
3000
C=O
1700
It is choice C….and the reason, click to find out
There is no way to know it was a cyclo but of the choices given, it had to have the C=O 1700cm-1 reading.
Phenol was an alcohol and the other 2, B and D did not have a F.Group. (note: the 3000 are is just the C-H for the CH2’s in the cyclo)

26. Another one?
Next slide, please

27. What compound is the most likely choice?
3000
C=O
1700
Is this:
A) ethylethanoate B) benzene C) hydroxybenzene
D) ethanoic acid

28. What compound is the most likely choice?
3000
C-H
3000
ethylethanoate
It is A….and the reason, click to find out
IUPAC name is ethylethanoate. There is no way to know the exact compound but if we get the functional groups right we can eliminate the ones it cannot be. Again, it had to have the C=O 1700cm-1 reading. C an D would have readings above 3000, as C was an alcohol and D a carboxylic acid B is just a hydrocarbon, no functional group in it

29. Question #3 Cyclopropanone B) propynol C) acrylaldehyde
D) propenoic acid